#GIS from a .NET developer's perspective

The tiles on the new Windows 8 Start Menu “tilts” when you press them, tilting toward the direction you click/touch them. This effect is also well known from the Windows Phone tiles. Unfortunately there is no out-of-the-box way of accomplishing this in either of the platforms. However for Windows Phone, a TiltEffect class is in the Silverlight Toolkit, so I figured I could just port that. Turns out it wasn’t that hard – all I pretty much had to do was change the manipulation events to use the new Pointer events instead, and will work with both mouse and touch.

To use it, first download the updated tilt effect class and add it to your project.

Next add a xmlns to register the namespace for use in XAML:

xmlns:sg="using:SharpGIS.Metro.Controls"

And lastly add the tilt effect to your control exactly like you would on the phone:

I’m a big fan of building controls. I love writing them, designing them, trying to make it work in as many scenarios I can while keeping them simple, extensible and most importantly reusable. In fact for the past 6 years, it’s all I’ve been doing full time (first ASP.NET and later XAML), and frequently in my spare time as well.

If you dabble in XAML, you have most likely already been building some controls, by going “File -> New -> User Control” in Visual Studio. You probably do this because you want to create a new page in your app, or you just want to encapsulate some of the UI in a separate section. Or perhaps it’s because you realize that this little tidbit can be used over and over again in your application. Or maybe you have even considered it can be used again and again across many DIFFERENT applications. If you have tried those two last categories (or if you will one day), this blog post is for you! It will apply to any of the XAML techs there is: WPF, Silverlight, Windows Phone and the future Windows 8 Runtime.

Despite the title in this blog post, User Controls are awesome. They are quick to throw together and reuse over and over - and there’s a lot of value in that. But what if I told you there’s another control type that has even MORE power, better performance, and can be way more flexible and reusable than a user control, and where the clean code will make most developers fall in love?

The thing is you already know this because you’ve been using them all the time: Button, ListBox, ItemsControls, Grid, StackPanel etc. are all controls harnessing the same power you can! And you have probably seen XAML styles that completely changed the look and feel of a control, without touching any of its code. To give you an idea of how powerful this is, look at this Silverlight Sample below. On the left you will see a ListBox binding to a list of planets. You have probably already done something like this. On the right, you see a solar system. But in fact this is ALSO a ListBox. And there is NO extra code involved here. It’s done entirely by restyling the template. Notice how selection and up/down keys work just like it does with the “normal” ListBox. So I got to reuse all the code that has this, and all I had to do was restyle the ListBox a bit. Something I could have done entirely in Blend without ever touching code.

So at this point hopefully I have won you over to learning more about Custom Controls (if not I’m amazed you have read this far :-).

The Anatomy of A User Control

To start, let’s first look at the anatomy of a typical UserControl and try and fully understand how that works first. Below here we have the XAML portion of our control that defines the layout. We’ll keep it simple and have a Grid with a Button inside it:

The things worth noting here are two things: “LayoutRoot” is defined in the XAML using x:Name, and we automatically get a variable by that name in code behind. Also the event handler hooked to the Button’s Click event is magically linked to the code-behind. All this is really handled by the compiler and the “InitializeComponent” call - a method that interestingly doesn’t exist here. The reason this works is really because this is a partial class as indicated, and Visual Studio creates a little ‘secret’ file under the covers for you. You can get to if if you right-click the method and select “Go To Definition”. Here’s what the contents of that file looks like:

You’ll notice that the LayoutRoot is defined here as internal, and it’s assigned using the “FindName” method.

This is one of the nice things about UserControls: A lot of the work is automatically done for you, but with Custom Controls you will have to do this yourself! (but this isn’t so bad considering the power you get!). And here’s the kicker: A UserControl is just another custom control!

The Anatomy of A Custom Control

A custom control doesn’t have a XAML and a code-behind component in the same way UserControl does. Instead it’s ALL code along with a default XAML template. You can consider the template the equivalent of the XAML in the User Control, but the important part to remember here is that this template can be changed by ANYONE, which is what I did to the ListBox in the solar system sample. Another thing to note is that since the template doesn’t have a corresponding code-behind where Visual Studio generates a partial class for you, any event handlers cannot be defined in the template. So how do we go about recreating the user control above as a custom control?

For Silverlight this is easy. Right-click your project and select “File -> Add New -> Silverlight Templated Control”. WPF and Windows Phone doesn’t come with this template so you’ll have to do it manually there, by creating a class and a generic template file. After you do this, you’ll notice two new files: First a simple C# class, and second a new file in \Themes\Generic.xaml. The second file is where you place all templates for all your controls in that assembly. It HAS to have this name and live in this folder for the custom control to pick up the template.

Below is what this template looks like. I’ve added the grid and the button inside the suggested border that was created for me.

First notice the TemplateBinding statements on the border. This is an important feature of controls. You can bind straight to a dependency property defined in your control code. Since a custom control inherits from “Control”, you automatically get Background, BorderBrush, BorderThickness and many other general properties from the inheritance. The great thing is that you can just write <my:TemplatedControl Border=”Red” /> and the border will automatically be bound into this template (and anywhere else where you have a TemplateBinding to that property). This beats UserControl, where to accomplish this in Silverlight you will have resolve to a hack by setting the DataContext of the control to itself, breaking the DataContext flow.

Second, notice that I didn’t add a click-handler to Button. If I did, this template would fail to load. We’ll hook the click handler up later.

First I declare the “TemplatePart” attributes on the control. They tell what parts (ie controls) are expected to be in my template. In this case LayoutRoot of type Panel (Grid is a Control), and ClickButton of type ButtonBase. These are not strictly required, but they help Expression Blend understand the template requirements when you later customize the control. I always declare the lowest needed type in the control hierarchy to make the template more flexible. For instance I use ButtonBase and not Button, because I only rely on the Click event which is declared on the ButtonBase base class. That way I don’t lock a user of the control into using “Button” but they can place ANY control that inherits from ButtonBase here. Same thing applies for the LayoutRoot, where I just need the Background property.

Next the control inherits from “Control”. Custom controls must inherit from this.

In the constructor I define the “DefaultStyleKey”. This tells the framework that I have a default template defined in Themes\Generic.xaml. If I didn’t the user would always have to explicitly defined a control template for the control.

Lastly, the most important part is “OnApplyTemplate”. This method is called when the control has loaded the template. This is our earliest opportunity to grab references to controls in the template, ie. the TemplateParts. In this case I grab a reference to the ButtonBase defined in the template. If it’s found, I’ll add a click handler to it. Also if a new template gets applied, I must remember to unhook from the previous instance (this is a rare scenario though, and you could probably get away with skipping that bit). It’s also important to note that Template Parts are always optional! So always do the null check anywhere you rely on a reference to a template part.

And that’s really it! I kept the sample simple, so it is easier to go through the individual parts of a control, therefore the differences between a custom control and a user control doesn’t really stand out. If this was all you needed to do, a custom control is probably overkill. But think of scenarios where you have a lot of code-behind that you want to reuse, but you don’t want to lock the design in. The major next parts you will want to add to this now is more Dependency Properties you can bind into the template, as well as VisualStates - ie. storyboards that triggers on certain events. The great thing about Visual States is that the code-behind doesn’t define the storyboard or what it does - only when it starts. This gives the user even more flexibility to customize the behavior.

Adding Visual States to the control

Let’s add some mouse over states to our control, and have the control animate when that happens. In the code-behind where we defined the TemplateParts let’s add two TemplateVisualState attributes:

This is really all the code we need. It’s pretty simple. If the mouse is over, trigger the MouseOver state, else trigger the Normal state. Note how we don’t really define what “MouseOver” looks like. That’s the job of the template. Let’s define that (you might already be very familiar with this when overriding templates - it’s exactly the same thing, except we get to define the default state):

So the changes here is adding a rectangle in the background that we animate into yellow when the mouse hovers over.

You now have a control that sets a background on some Panel when some ButtonBase is clicked, as well as running an animation on MouseEnter/Leave. This could serve the purpose for quite a lot of controls, without you having to rewrite the code!

Here’s a few resources you will want to read if you want to learn more about this:

In Silverlight/WPF and WPF there’s today a WebClient class for making webrequests. Under the covers it creates HttpWebRequest for sending the request and HttpWebResponse for handling the response coming back from the server. In an older blogpost, I created a custom WebClient for WP7 that added support for GZIP compression this way.

However the default WebClient doesn’t actually create an “HttpWebRequest/Response” object, since this is an abstract class. What it really does is create internal subclasses that is then used. This is handled by the WebRequestCreator static class which really creates these classes for you. However a less-known feature is that you can actually tell your application that you want to use a specific client for handling the web request and responses for specific domains. You might actually already have fiddled with this in Silverlight if you wanted to explicitly let Silverlight handle the web requests instead of handing it off to the hosting browser control using the following statement:

…and ALL web requests to http or https websites, will be routed through your MyHttpWebRequest class (which you granted still have to build). This class could just wrap the built-in ClientHttpWebRequest and you can modify/add/remove/decode stuff to your pleasing. This is awesome for test framework mock ups, but it’s also really neat for the GZIP WebClient control I mentioned earlier. Why? Well to use my GZipWebClient, you would have to go through all your code and change:

WebClient client = new WebClient();

to

WebClient client = new SharpGIS.WebClient();

This can be a lot of work, and even if you did that, what about all the 3rd party libraries you use, where you can’t go in and change that? Well enter RegisterPrefix! Do you want GZIP support in Hammock? Done! Do you want GZIP support in RestSharp? Done!* Do you want GZIP support in [insert favorite API here]? Done!

*Update: Actually this might not be the case. Some libraries uses some features that either is not possible to implement with the custom HttpWebRequest (like UserAgent), and I didn’t add support for Cookies as well. Both of these are for instance used by RestSharp.

So I went ahead and updated my Nuget Package for the GZipWebClient, so go grab v1.1, and you will only have to add 1-2 LINES OF CODE, and just execute it ONCE (for instance at startup), and EVERYTHING WebClient will be enhanced with GZIP compression. You can also grab the updated source on the original blogpost here.

I really have to give some credit to my co-worker Anargyros for this one, who mentioned that it might be possible to do it this way. Go follow him on twitter and give him some twitter love @tomahawk1187

…where Thumbnail is a low resolution image that loads fast, while the full resolution Image is being downloaded.

If you just want to use this control and don’t want to learn how to create a custom control, skip to the bottom to download the source for both Windows Phone and Windows 8 Runtime.

First off, we’ll create a new Windows Phone Class Library project and name it “SharpGIS.Controls”. (or whatever you like)

Create a new folder “Themes”, add a new XAML resource file and name it “Generic.xaml”. Make sure the “build action” for this file is set to “ApplicationDefinition”.

We will want to define the default template in this file for our control (if you are used to making User Controls, this is essentially where the XAML for custom controls go instead).

In the xaml we will want two things: An image for displaying a fast-loading thumbnail at first, and a second image for displaying the high resolution image. Also we will use a Grid around them to group them together. Generic.xaml should look something like this:

Notice the “CacheMode” property. This is important to set, to get proper performance when zooming and panning the image. If you have a Windows Phone try the official Facebook app, open an image and you will see what I mean (if you worked on that app PLEASE fix this).

Next up is the actual code. Add a new class, and name it “ImageViewer”. Make it inherit from ‘Control’. Add a constructor and use the “DefaultStyleKey” to tell Silverlight that you have a template defined in Generic.xaml that it should use.

We should now be able to use this control in a Windows Phone project. Add a new Windows Phone Appplication project to your solution, right-click the ‘references’ and select “add reference’. Pick the SharpGIS.Controls project.

You should now be able to use some XAML like this to display an image:

Go back to the code and override OnApplyTemplate(). This code executes when the template from Themes\Generic.xaml has been loaded, and it’s your chance to grab any reference to the elements in there and ‘do something’ with them. In user controls you would often set the event handlers directly in the xaml. With templates on custom controls, you will have to hook these up in code-behind during OnApplyTemplate().

Here we will hook up for the manipulation events, as well as assign a transform we will apply to the element when these events trigger.

The ManipulationDelta event triggers as you move. It will give you information about how much the user dragged, and how much he/she pinches, as well as the center of the pinch. Unfortunately the pinch scale amount is shown as separate X and Y directions, and no ‘Uniform Scale’ is shown. This makes it hard to get a good average of the scale, and you would have to pick just one of them (one could be pinching and the other stretching).

I’ve found that defining the amount you’re scaling is the change of the length of the diagonal of the boundingbox of all the touch points works well. Errrrrrr, that might have sounded confusing. Let’s use a picture instead. The orange circles below are touchpoints, and the rectangle is the box that encompasses all of them (2 or more points). The blue line is the diagonal length of this box. So the amount of scaling = length_Before / length_After.

We don’t get the actual touch points in the manipulation events. So instead I start with a simple box that I define as 1x1 and track the scaling of it. The diagonal length of that box at the beginning is the square root of 2, which we will define in the ManipulationStarted event. We also add a property for tracking the center of the box.

So all that’s left is listen to the ManipulationDelta event, and update the transform on the grid. This consist of comparing previous origin to the new, as well as calculating the scale factor based on the box diagonal. Also note that when you apply scale, this is means you’re scaling out and away from the upper left corner. To offset this, you will need to add some additional translation to the image, as shown below:

Now what’s left is to turn off the thumbnail when the image has loaded, as well as raise loaded events for the high resolution image, so that you can display a progress bar while you wait for the image to load. I won’t go into details on this, but in OnApplyTemplate, simply grab the Image template child, and listen for the ImageLoaded event.

I’ve packaged it all up in one complete control, as well as a sample showing how this would be used in an application where you would navigate to a page with this control on it.

Perhaps I’m missing something here, but writing a simple text file and reading it back in is surprisingly complex in Windows 8 Metro style apps/WinRT. In .NET there’s a one-liner for such a trivial task, but I haven’t been able to find anything like that in WinRT. So to save you the trouble, here’s my utility methods for this:

Notice that there is also a WriteTextAsync(string,string) method. However this seems to throw an exception if the file doesn’t already exist, so I’m not sure how useful that is at this point, considering this will be a-chicken-and-the-egg scenario. So for now, the code above is still useful for writing files, and of course also for understanding the more low-level file operations.

If you are making a video app for Windows 8 Metro there are 6 lines of code you simply MUST add. The feature you get for such a little effort will make your jaw drop (well at least mine did). Here’s what the experience is like:

Imagine you are watching a video on your Windows 8 PC or Tablet:

But watching TV on the PC kinda sucks. So the first thing you do is to swipe from the right to bring out the Windows 8 Charms menu:

You hit the “Devices” charm, and voila, all your media clients show up under “Play To”. Now you just pick your PlayTo enabled TV/Device, or even any Windows Media Player running on Win7 or 8:

And the next thing you know this happens:

Voila! The Christmas cheer is now playing on the TV for everyone to enjoy. And I can continue to use the playback controls on my PC to play, pause, fast forward, rewind etc.

And that’s ALL you need to do! No need to build any UI around this. No need to figure out how to stream various types of video streams (as far as I can tell both local files and remote video streams works with this). And yes this can even be extended to work with images and music as well.

So those 6 lines of code should ALWAYS be in your video player app, and you’ll make a lot of people very happy (Netflix and Hulu are you reading this?).

In the Windows 8 Developer Preview there is a limitation on what devices this works on for now though. From the white paper:

Only the below list of Play To receivers are supported in the Developer Preview. For scenario testing/validation from apps the easiest solution is to use Windows Media Player on another PC on the same Private Network. Other available devices are:
· WD Live Hub (firmware version: 2.07.17)
· Onkyo AV Receivers (Windows 7 Certified models)
· Windows Media Player (on Windows 7 and the Windows Developer Preview)
· Metro style apps with Play To Receiver functionality

Since Windows Media Player supports this, it’s pretty easy to test for yourself with a second Win7 or 8 PC. Here’s how to configure Windows Media Player as a Play To receiver:

· Open Windows Media Player and navigate to the Stream menu
· Select Allow remote control of my Player

If both PC’s are on the same private network and have sharing enabled, that’s really all you need to do. Windows 8 will automatically pick it up and add it to the list of Play To devices.

If you have problems making this work, I suggest you first read the white paper, since this also covers known issues and how to correctly configure your devices on your network.

So Silverlight 5 finally hit the web this week, and there’s a lot of cool new features – some of which I’m very excited about, but probably the biggest feature ever added - the 3D XNA API that allows you to render directly to the graphics card using the new DrawingSurface API - I’m not so excited about any more. Why? Because between the RC and RTM release this feature has been heavily crippled. Here’s an excerpt from the recent blogpost describing the change:

[…] you may experience security errors with Silverlight 5 RTM when you want to use the wonderful new 3D feature. In fact, some graphics drivers may allow malicious code to execute. That may lead to an unwanted hard reset or a blue screen.

Starting with the beta version, to protect users for this kind of trouble, we initiate a first scenario where all Windows XP Display Driver Model (XPDM) drivers on Windows XP, Windows Vista, and Windows 7 will be blocked by default. Permission will be granted automatically in elevated trust scenarios and Windows Display Driver Model (WDDM) drivers will not require user consent at run-time.

But as always, features, including security features, continue to be refined and added during post-beta development.

And for the RTM version, there were a number of approaches considered to further improve security and stability, but the solution to block 3D in partial trust by default was the best option for this release. Permission is still granted automatically in elevated trust scenarios.

So already during beta and RC, we had a security limitation that certain old type of display drivers would disable this mode, unless you ran in full trust. Considering the uptake on WDDM, I didn’t consider this limitation a big problem. After all technology moves forward, and old tech sometimes needs to be left behind. After all one of the reasons for moving to the WDDM model was to stabilize the system.

Post RC this limitation was increased to be across all systems and display drivers. I was lucky enough to get a heads up about this a little over two months ago, but I feel truly sorry for the people who had been betting on this feature and are ready to release their 3D apps now that RTM has hit. The communication of this limitation shouldn’t have stayed within a small group of people under NDA, but should have been communicated out to the community. It probably didn’t happen because it could probably have fueled the “Silverlight is dead” fire (and rightfully so). I would have like to have seen a more open discussion of this, similar to what the Windows 8 team has on their “Building Windows 8” blog. In June the Silverlight Team had promised that the DoS issue that beta and RC had, would be mitigated and developers were counting on this feature.

So I’m not saying that security shouldn’t take precedence, but I would have like to see a more focused effort to either improve the experience – either by dedicating a lot of resources to weed out the security issues, delay the release, or at the very least make it easier for users to opt in. – perhaps communicating that this is a temporary limitation and will be resolved in a GDR update. Considering the late change, I get that the Silverlight team didn’t have much time to turn around and improve this experience. However, there’s already a feature like this for when you want to use the webcam that works much better. When I in Silverlight want to use the webcam, I ask the user for permission, and I’m met with the following dialog:

It’s pretty easy and it’s clear to the user what he/she needs to do. I wish we had gotten a similar dialog for enabling 3D. Instead, now in Silverlight, all you can do is detect that the 3D capability is blocked and try and describe to the user what to do. These are the steps you would have to guide them through:

Right click on your Silverlight plugin.

Click the “Silverlight” option.

Go to the permissions tab.

Find the domain that hosts the .xap file (which might not be obvious since it could be different than where the website is located).

When you find the entry (and this list could be quite long), you have to click “Allow” – no indication here what that really means for the casual PC user (how do you use a blocked display driver?!??).

Was this really the best experience we could have gotten? It’s actually easier to ask the user to install the app in full trust (which is an even unsafer mode), than the above experience. I’m sure Average Joe would not like this experience, have trouble finding this out, and probably even be scared to death to do anything he’s not used to doing (I know my parents wouldn’t ever get through this). So all you are going to see on your website is a plugin container that just sits there unused, and not getting to show how awesome this thing could really have been.

It’s such a shame, because this API can vastly improve rendering performance in many scenarios, and opens up an entirely new group of 3D based web based apps. And it would have worked across a wide range of browsers (more so than WebGL at this point).

The 3D capabilities are still great for out-of-browser apps and internal line-of-business apps where it’s a lot easier to deploy an app in full trust (especially with SL5’s new in-browser full trust feature). But are that really the type of apps where you expect to be using the 3D feature a lot?

I’ve lately been playing a lot with augmented reality on my Windows Phone and my Windows 8 Tablet. Both of them feature an accelerometer, compass and gyro as well as a back facing camera. This makes it possible to overlay various XAML elements on top of the camera feed, and have them move with the camera as you rotate and orient the device. As an example of this, you can try the AR view in my GuidePost app for Windows Phone (and while you’re at it, make sure you throw some good reviews in there ;-). The app uses the exact approach described here.

To do augmented reality in XAML, I built a custom panel control that automatically handles the placement of all it’s child elements based on the device’s orientation. I used an attached property to define what ‘direction’ an element is supposed to be placed in. You already know this from Grid and Canvas, where for instance Grid.Row=”1” places it in the second row of the grid, or Canvas.Top=”100” places the element 100 pixels from the top of the canvas. In my case I use ARPanel.Direction to define a Point where the first value is the azimuth (ie up/down) and second value is the compass heading.

Note: Since you probably know the Grid panel very well, to help explain how this custom panel works, you’ll sometimes see a “How Grid uses this” section, to explain how the grid control would do something similar.

Here’s what the four cardinal directions and up and down would look like in such a custom panel that we’ll be building:

MeasureOverride is called first, and it should go through all the child elements and tell them “Hey I got this much space available - how much of that do you need?”.

How Grid uses this: If a grid is 150x100 and has two rows and two columns, it would look at the row/column of the element then say “I got this much 75x50 space in this row/column - how much do you want”. If all the row/column sizes are auto, it would send the entire size of the grid itself to the child.

ArrangeOverride is called second. This goes through all the child elements and tells them “I heard you want this much space (as indicated by child.DesiredSize), but I can only give you this much, and this is the area where you get to place yourself”.

How Grid uses this: After going through the measure step, Grid would now determine what the size of each row and column needs to be based on the child desired elements (unless the cell sizes are fixed), and then calculate the rectangle for each cell distributed based on the row/column definitions. These rectangles are then used to place the child elements in the row/column they are set to be placed in.

Now first let’s implement MeasureOverride, since this is very simple in this scenario. Each child element can render itself anywhere around you in a 360 degree space, so we don’t need to limit the child elements to any size other than the size of the panel itself. So we’ll ask each child “Given all of my space, how much would you want to use?”. This is simply done by going through each child element and calling Measure on them with the full size available (after each measure call, you will notice that child.DesiredSize will now be assigned). We end the call by returning the size that the panel needs to it’s parent - in this case we want to use the entire space available to us:

MeasureOverride is called whenever the panel’s size is changing or anything else that would affect the size of the child elements. You can trigger this yourself by calling panel.InvalidateMeasure(). In our specific case we don’t ever need to call this, and the measure will only get invalidated when the panel’s size changes - for instance when page orientation changes, and luckily this is all done for us. (Grid would call InvalidateMeasure if the row/column definitions change)

Next up is ArrangeOverride. Now that we measured all the elements, we need to determine where to place them on the screen, and how much space we need to give each of them. We’ll basically go through each element and call Arrange(rect) on them, where the ‘rect’ is the rectangle it gets to render itself in. Ie. the top-left corner is where the area starts, and the width/height is the size it gets. It’s then up to the child to make use of that area. In our ARPanel case, the top/left corner would constantly change as the orientation of the device changes, but we’ll keep the width/height to the desired size of the element. So the method will look something like this:

I subtract half the width/height of the size to center the element on top of ‘centerX’ and ‘centerY’, which we will still have to determine;

So this is basically our panel implementation. The tricky part is figuring out the “TODO” part of the above code based on the motion sensor. So basically, given a device orientation, what is the screen location of a direction?

Warning: I spent quite a lot of time on getting this part right. It’s a lot of vector math, and some of it I came to using pure trial and error :-). Therefore I might not even completely get all of it myself, but it works, and it works great. I’m pretty sure the math is right, but if I don’t go too much into detail in some of the math, just think of it as magic that works and use the code as is :-) For those of you who are used to XNA 3D Game programming, I’m sure this will all make sense to you though, since this is very similar to how you control a camera in a 3D environment.

The motion sensor

The Motion Sensor is what will be driving the placement of the child elements. Motion is a combination of Compass, Accelerometer and optionally Gyro. The Gyro is not required, but it makes the result A LOT better - especially when you rotate the device fast. Future certified Windows 8 tablets will require all 3 sensors, but for Windows Phone, only some of the newer Mango phones has a gyro, and I believe the Dell Venue Pro compass sensor doesn’t work. Future cheaper Windows Phone devices might not come with any sensors available, so beware that your app might not work on all devices, and you should check for the capability and notify the user if it’s missing.

On Windows Phone you can check whether the motion sensor is supported using ‘Microsoft.Devices.Sensors.Motion.IsSupported’. I didn’t find an equivalent property in Windows Runtime though. The following code starts reading from the sensor for both WinPhone and Windows Runtime:

When we get a new reading, all we have to do is tell the panel, that the current child placements are invalid. We don’t want to do too much work here, because the motion sensor can be triggering much more frequent than the layout cycle, so all we do here is flag the panel for arrange and clear any parameters that could be affected by the orientation change (like the attitude which I will get back to):

The Motion sensor gives us a Matrix that defines the rotation of the device relative to up and north directions. The screen has a different coordinate system that is relative to the upper left corner of the screen. When the screen changes between landscape and portrait mode, this coordinate system changes relative to the motion sensors coordinate system, and it’s important to take this into account. Lastly we need to define a field of view for the screen. If you are overlaying elements on top of the camera, the field of view must match the field of view of the camera. Think of it as how ‘narrow’ the view is. A lens that is zoom in far has a small field of view, and only few elements will be visible on the screen, whereas a wide angle lens will have more child elements on the same screen. I found that on my phones a FOV of roughly 35 degrees seems appropriate. So to sum up we need 3 things: The orientation of the sensor, the orientation of the view (screen), and the parameters for the camera (the projection).

First the view, which is slightly different between phone and runtime (probably because the ‘natural’ orientation on a phone is portrait mode, whereas on a PC it’s landscape mode, so the sensors are mounted different).

Now we need the attitude of the device based on the current sensor reading (this was the value we reset in reading changed event). We rotate this value so it matches the rotation of the XNA coordinate system as well.

We don’t have any radius, but I found always using a value of ‘10’ (which is 10 units down the ray) works pretty well. So… we are now pretty set to implement the ArrangeOverride method.

Arranging the child elements

When arranging the children, we need to first check if it’s inside the screen view, and next where on the screen that is. I’m using the BoundingFrustum to do this. Think of the bounding frustum as the space the camera can see, and is a cone the expands out from the camera We can set this up using:

If we now build a BoundingSphere around each element, we can use the .Contains method to see if that element is within the frustum. So we first grab the Point object from the element, and build a bounding sphere. If it is inside, all we have to do is call the Project method above to get the screen location, and lastly call Arrange on the element.

Congrats on making it this far - because we are actually pretty much done! All there’s left to do is put it all together, reset the used orientation parameters when needed and define the attached property for use on the child elements. And I already did all that for you!

You can download the control library together with a sample app here. There’s both a Windows Phone and Windows 8 Metro version included. You can use the code as you like, but if you improve on it, I would LOVE to hear about it. I didn’t include a camera background in the samples, so I’ll let that bit be up to you to add (you can use a VideoBrush as the background for that).

Note that for Windows Runtime, I have only tested this on the Samsung Build Tablet. I have no idea how this behaves on a device without a motion sensor (probably crashes, but let me know in the comments below). Also the Build tablets have a buggy fusion sensor firmware, which means that readings are fired VERY infrequent, making it almost useless for anything but rough testing.

Also note that the compass sensor can get very confused when you are inside - especially if the building has a metal frame construction. If directions seem off, try going outside away from anything that confuses the magnetic field.

Tim Greenfield - Silverlight 5 vs. WinRT comparisonA follow-up to the link above with a comparison of namespaces, members, types and differences between SL5 and WinRT. This is an amazing list if you want to get into the details when reusing code between the two frameworks.

With GPS receivers integrated in phones, tablets and PCs, a lot of apps are being built that displays your current location. Usually the GPS API’s that these devices come with provides you with two values: Longitude and Latitude in decimal degrees, and often an estimated accuracy in meters. Most apps just display these values as is, without considering formatting or number of significant digits. This blog post attempts to show how this could be done. I’ll use C# for this, but it should apply to any device, language and API out there.

Formatting

Some people think of longitude as the “X” in an ordinary flat X,Y the coordinate system and latitude as Y/Up/North. It’s technically not correct because this is not a flat coordinate system, but a ‘polar’ coordinate system - I do however understand this simplification and I think of it the same way internally when working with code that deals with any type of coordinate system. However, how things work internally and how they are displayed are two very different things. An example of this are date objects: They are usually stored and handled as a ‘tick’ number, but we never display it like that. Geographical coordinates are the same way. They have one way they are stored in memory, and a completely different way to be displayed.

First of all lets get the order out of the way: If you still think of longitude as the ‘x’ you probably want to display this value first. However it’s commonly agreed upon that latitude is displayed before longitude.

Next are negative coordinates. Instead of showing a latitude/longitude as for instance -117,34 you would write 117°W 34°N. So we prefix the coordinate with N/S and E/W depending on weather the coordinate is positive or negative. So in C# this could look like this:

Now this is still decimal degrees. A more ‘proper’ format would be to use the degrees, minutes, seconds (DMS) format . Some people do prefer decimal degrees though, so you might want to make this a configurable option. But if you expect people to be using this coordinate to plot a position on a map, you are better off using DMS, since this is the format maps uses along its edge - and it also looks prettier. Degrees are denoted with a °, minutes with a single quote ' and seconds with a double quote ". For example 117°W 23' 12.34”

To make this conversion you will first show the integer part of the degrees. Take the remainder multiply by 60, and you’ll get the minutes. Lastly take the remainder of that and do the same, and you got the seconds (and you can display the seconds with decimals, but see the part on ‘accuracy’ next). Below is what that will look like in C#:

Often I see a location displayed as for example -117.342817243 , 34.212381313. When I see this many digits I instantly think ‘oooooh that’s a very accurate location’. But this is very misleading. In college, several of our professors would fail our reports if the end result displayed more digits than the accuracy of the input data. The same thing applies here. If your GPS receivers accuracy is 1000m, how many digits should you display, and how many meters is one second?

First a little about the size and shape of earth: While earth is not a perfect sphere, it’s fairly close to an ellipsoid (this is still an approximation though). It’s widest at equator, and smallest (flattened) between the north and south pole. So in ellipsoid-speak the parameters are:

So back to the question: How many meters is one second? This is pretty easy to determine for latitude, but unfortunately this is not a straightforward conversion for longitude, since this changes with the latitude. Let’s first start with the simpler latitude:
First we need the circumference of Earth along a meridian (a line that goes north/south) and Equator:

For simplicity let's stick with a rough average of 40mio meters, since this is not going to really matter for the end result.
From that we get:

Horizontal length of one degree at Equator or along a meridian:
40,000,000 / 360 = 111,111m
Horizontal length of one second at Equator or along a meridian:
111111.111 / 60 minutes / 60 seconds = 31m

So from that we get that we should never display any decimals on seconds unless our accuracy is better than 31 meters. And we shouldn’t display more than one decimal unless the accuracy is 3m or better (which never happens with standard GPS equipment). Similarly if we are using decimal degrees instead of DMS for display, how much does the n'th digit matter at Equator or along a meridian?

So in this case we will only show 7 digits if accuracy is better than 40m, and probably never more than 7 digits.

Latitude always goes along a meridian, so the number of significant digits doesn't ever change with your location. But the length of one degree at a longitude changes with the latitude you're at.
The radius of a longitude at a given latitude is: cos(latitude)*mR*2*PI.
At 34 north or south that would be: 33,168,021m. So here the number is roughly 3m instead of 4m, meaning you are more likely to show more digits on the longitude portion for the coordinate, the further north you go. In general practice however, this is not going to matter too much, since it only gets better. so to keep it simple we’ll just stick with the same conversion at all latitudes.

Bringing it all together

So let’s bring all this together into a C# ValueConverter you can use for binding against a GeoCoordinate object returned by the GeoCoordinateWatcher in .NET and Windows Phone. A converter parameter is used for choosing whether you want DMS or decimal degrees as output.